Fall detection

ABSTRACT

Proposed are concepts for distinguishing between fall events and non-fall-events for different sub-groups of subjects within a monitored group (i.e. monitored population) of subjects. It is proposed that an entire group/population of monitored subjects may be partitioned into sub-groups, each sub-group consisting of a plurality of members (i.e. subjects) having a certain property value or characteristic unique to that group. A respective decision value may be determined for each sub-group, wherein the decision value for a sub-group takes account of a previously obtained false fall detection rate for that sub-group.

CROSS-REFERENCE TO PRIOR APPLICATIONS

This application claims the benefit of European Patent Application No.19215279.1, filed on 11 Dec. 2019. This application is herebyincorporated by reference herein.

FIELD OF THE INVENTION

The invention relates to the field of fall detection, such as falldetection employed in Personal Emergency Response Systems for example.

BACKGROUND OF THE INVENTION

Personal Emergency Response Systems (PERS) enable elderly and disabledpeople to live independently by summoning immediate help when an adverseevent, such as a fall, occurs. The use of such systems allows people whowould otherwise require round-the-clock care to live in their own homes,and reduces their care costs.

Some PERS systems rely on subject input, such as a button press, toraise an alarm when assistance is required, but this means that asubject is unable to access immediate medical help in the case of thesubject becoming unconscious as a result of a fall, when such help ismost critical. There is also a risk that the subject may fall in such away that, while remaining conscious, he/she is unable to activate thealarm.

For these reasons, automatic fall detection systems have been developedthat can summon help without needing subject input. These detectionsystems generally come in the form of wearable devices that containsensors to monitor subjects' movements and processors that decidewhether a detected movement is the result of a fall. However, currentautomatic fall detection systems are not able to distinguish withcomplete accuracy between falls and movements that occur during everydayactivities. For example, fall detection accuracy is particularly low inwrist-worn devices.

In order to avoid failing to detect genuine falls, fall detectionsystems may be configured to have a high false detection rate (i.e. therate of non-fall events classified as fall events) so as to minimize thenumber of falls that are not detected as such. A high false alarm ratenecessitates the use of a cancel function to avoid summoning helpneedlessly; subjects can activate this function when the fall detectorincorrectly detects a fall. However, some subjects may find the cancelfunction difficult to use, become panicked when a false alarm occurs andforget to activate the cancel function, or accidentally activate thecancel function when a genuine fall has occurred.

There is therefore a need for fall detection concepts that are able todetect most actual fall events while keeping the number of false alarmsto a minimum. However, these two requirements are contradictory, becauseit is typically only possible to increase the detector's true positiverate (i.e. detection of actual falls) at the cost of having a higherrate of false alarms.

SUMMARY OF THE INVENTION

The invention is defined by the claims.

According to examples in accordance with an aspect of the invention,there is provided a computer-based method for distinguishing between afall event and a non-fall-event for a plurality of subjects.

The method comprises: obtaining first and second false fall detectionrates for first and second sub-groups of the plurality of subjects,respectively, and wherein the first and second sub-groups comprisesubjects with first and second differing values of a property of theplurality of subjects, respectively; determining, based on the firstfalse fall detection rate, a first decision value for distinguishingbetween a fall event and a non-fall-event of subjects of the firstsub-group; and determining, based on the second false fall detectionrate, a second decision value for distinguishing between a fall eventand a non-fall-event of subjects of the second sub-group.

Proposed are concepts for determining different decision values (e.g.threshold values) for distinguishing between fall events andnon-fall-events for different sub-groups of subjects within a monitoredgroup (i.e. monitored population) of subjects. In this way, a propertyor characteristic specific to a group of monitored subjects (which mayinfluence a false fall detection rate) may be accounted for, therebyproviding for adjusted or tailored event classification that leads toimproved fall detection accuracy. Embodiments may therefore dynamicallydetermine and adapt decision values (used for fall event detection) todifferent sub-groups of a monitored subject population.

Embodiments may therefore achieve a larger true positive detection rate,on average, while keeping an overall false alarm rate below anacceptable maximum.

For instance, it may be proposed that an entire group/population ofmonitored subjects is partitioned into sub-groups, each sub-groupconsisting of a plurality of members (i.e. subjects) having a certainproperty value or characteristic unique to that group. A respectivedecision value may be determined for each sub-group, wherein thedecision value for a sub-group takes account of a previously obtainedfalse fall detection rate for that sub-group.

By way of example, location-specific factors may contribute to thenumber of false alarms (i.e. false fall detections). For instance, ithas been observed that variations of atmospheric pressure, e.g. due tostorms, may have a significant impact on the false alarm rate of anaffected location. Such factors may therefore be taken account of bydefining sub-groups according to subject location and then determining adecision value for each sub-group, the decision value for each sub-groupbeing determined so that a false fall detection rate for subjects ofthat sub-group equal a predetermined acceptable rate for example.

By way of further example, subject activity, fall risk, age, weight ormedical conditions may be used to define sub-groups of monitoredsubjects.

Proposed concepts may therefore an improved (higher) true positivedetection rate across an entire monitored group/population of subjects,while maintaining an overall false alarm (i.e. false fall detection)rate below a predetermined acceptable maximum. Such concepts may, forexample, dynamically adapt a classification decision value to differentsub-groups of the monitored group/population of subjects. A wide-rangeof properties of the subjects may be used to define sub-groups of themonitored group/population of subjects. In this way, proposedembodiments may take account of a variety of differentproperties/characteristics of subjects that may impact false alarmrates.

Unlike conventional fall detection concepts, which may employ a fixedfall detection decision value for all monitored subjects, proposedconcepts can determine different fall detection decision values fordifferent sub-groups of the monitored subjects. Such sub-groups may bedefined based on properties or characteristics of the monitoredsubjects, e.g. by splitting the monitored subjects into sub-groups basedon their location or age. For instance, each subject of a firstsub-group may have the same first value of a property, and each subjectof a second sub-group may the same second value of the property, thefirst value being different from the second value.

Embodiments may ensure that the fall detection decision values arewithin certain limits so to ensure a minimum detection probability.However, the variation of decision values across different sub-groups ofa monitored population may be such that the average over the sub-groupsresults in an improved fall detection accuracy (i.e. a reducedlikelihood of false alarms being generated when a subject has notactually fallen).

In some embodiments, determining the first decision value may comprisedetermining a value at which a false fall detection rate for subjects ofthe first sub-group is equal to a predetermined rate, a false detectionrate being a rate at which non-fall events are determined to be fallevents. For instance, the predetermined rate may be defined so as tomaintain a predetermined minimum detection probability value. In thisway, the value of a decision for a sub-group may be optimized so as toensure a minimum detection sensitivity is achieved whilst ensuring thatfalse detections are reduced or minimized.

Further, the first and second decision values may be determined so as tomaintain an overall false fall detection rate across the first andsecond groups remains constant whilst the an average true positivedetection rate is increased.

The first decision value may be associated with each subject of thefirst sub-group. Similarly, the second decision value may be associatedwith each subject of the second sub-group. Put another way, the firstsub-group may be assigned the first decision value, and the secondsub-group may be assigned the second decision value. This may enable thedecision value for each monitored subject of the entire monitoredpopulation to be individually assigned/allocated based on the sub-groupit belongs to.

In some embodiments, the first and second false fall detection rates forfirst and second sub-groups may be obtained from a separate and/orremotely-located information source (e.g. a fall detection system,database of fall cases, subject monitoring system, etc.). In this way,embodiments may be configured to simply receive information about falldetection rates for a monitored population of subjects, and then employthe received information to determine decision values for differentsub-groups of the monitored population. Proposed embodiments maytherefore information from existing fall monitoring or detection systemsin order to dynamically and flexibly determine different decision valuesfor distinguishing between fall events and non-fall-events of differentsub-groups of monitored population of subjects. Embodiments may increasea value of existing/conventional fall detection systems by providingmodified and/or extended functionality.

Conversely, other embodiments may be configured to determine the firstand second false fall detection rates for first and second sub-groups.For example, an embodiment may comprise: identifying the first andsecond sub-groups of the plurality of subjects; monitoring the pluralityof subjects to detect potential fall events; for each detected potentialfall event, distinguishing between the detected potential fall eventbeing a fall event and a non-fall-event based on a fall detectionalgorithm employing a decision value; and determining first and secondfalse fall detection rates based on the identified first and secondsub-groups and the results of distinguishing the potential fall eventsbetween being fall events and non-fall-events. Thus, rather than relyingon information about monitored subjects from a separate monitoringsystem, embodiments may be configured to monitoring subjects in order todetect potential fall event and determine false fall detection rates.For instance, an embodiment may comprise a subject monitoring system ora PERS system with improved or extended functionality provided as aresult of the proposed concept(s). Purely by way of further example, anembodiment may comprises a PER system that is adapted to definesub-groups of a monitored population of subjects and to implementdifferent fall detection decision values for the different sub-groups.

According to some embodiments, the fall detection rate for a sub-groupof the plurality of subjects may be based on a ratio between a number offalse fall detections for the sub-group and the number of subjects inthe sub-group. In this way, embodiments may take account of sub-groupsize (i.e. the number of members of a sub-groups), so as to provide arelatively consistent and comparable measure across sub-groups ofdiffering sizes.

By way of example, wherein the property of the plurality of subjects maycomprises: location; activity; fall risk; age; medical condition;weight; gender, diagnosis; disease; prescription; equipment/aids used,etc. Various properties or characteristics of the monitored subjects maytherefore be used to group the subjects into sub-groups, and theproperties/characteristics may be physiological (and fixed or slowchanging) and/or situational (and thus variable with respect to time).Such properties/characteristics may directly (or indirectly) influence afalse fall detection rate, and so proposed embodiments may account forthis by defining groups based on these properties/characteristics andthen determining different decision values for the different groups.Proposed embodiments may therefore leverage a realisation that aproperty of a monitored subject may influence or affect a false falldetection rate. By modifying a decision value to account for such adependency on the property, embodiments may adapt the decision value toprovide more accurate fall detection (e.g. enable the distinguishingbetween a fall event and a non-fall-event such that false fall detectionrate is reduced whist ensuring that true positives are detected). Inthis way, the decision value may be customized to a group of subjects toensure that the false alarm rate for that group of subjects is at anacceptable level.

According to examples in accordance with an aspect of the invention,there is provided a computer program comprising code means forimplementing the method of any previously described method when saidprogram is run on a processing system.

According to a concept of the invention, there is provided a system fordistinguishing between a fall event and a non-fall-event for a pluralityof subjects. The system comprises: an interface component adapted toobtain first and second false fall detection rates for first and secondsub-groups of a plurality of subjects, respectively, and wherein thefirst and second sub-groups comprise subjects with first and seconddiffering values of a property of the plurality of subjects,respectively; and a decision value calculation unit adapted todetermine, based on the first false fall detection rate, a firstdecision value for distinguishing between a fall event and anon-fall-event of subjects of the first sub-group, and to determine,based on the second false fall detection rate, a second decision valuefor distinguishing between a fall event and a non-fall-event of subjectsof the second sub-group.

It may be preferable, to define the (at least) two decision values suchthat the total false fall detection rate, of all the combined groups, isbelow a certain predetermined acceptable value. Such optimization of thedecision value may be undertaken so as to maximize the lowest falldetection probability (i.e. sensitivity) over the groups, or to maximizetheir average, etc.

There is also proposed a system for detecting a fall of a subject,comprising: one or more sensors for obtaining movement data responsiveto a subject's movement; and the system for distinguishing between afall event and a non-fall-event for a plurality of subjects according toa proposed embodiment, further configured to: receive the movement datafrom the one or more sensors; detect a potential fall event based on themovement data; determine which sub-group the subject is a member of; andclassify the detected potential fall event as a fall event or a non-fallevent by comparing information associated with the fall event with thedecision value for the sub-group the subject is determined to be amember of.

These and other aspects of the invention will be apparent from andelucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention, and to show more clearlyhow it may be carried into effect, reference will now be made, by way ofexample only, to the accompanying drawings, in which:

FIG. 1 depicts a variation in false fall detection rate over time forfour different states in America;

FIG. 2 shows a correlation matrix between the false alarm rates of thefour different states in America;

FIG. 3 is a flow diagram of a method for distinguishing between a fallevent and a non-fall-event for a plurality of subjects according to aproposed embodiment; and

FIG. 4 illustrates a system for detecting a fall of a subject accordingto an exemplary embodiment.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The invention will be described with reference to the Figures.

It should be understood that the detailed description and specificexamples, while indicating exemplary embodiments of the apparatus,systems and methods, are intended for purposes of illustration only andare not intended to limit the scope of the invention. These and otherfeatures, aspects, and advantages of the apparatus, systems and methodsof the present invention will become better understood from thefollowing description, appended claims, and accompanying drawings. Themere fact that certain measures are recited in mutually differentdependent claims does not indicate that a combination of these measurescannot be used to advantage.

Variations to the disclosed embodiments can be understood and effectedby those skilled in the art in practicing the claimed invention, from astudy of the drawings, the disclosure and the appended claims. In theclaims, the word “comprising” does not exclude other elements or steps,and the indefinite article “a” or “an” does not exclude a plurality.

It should be understood that the Figures are merely schematic and arenot drawn to scale. It should also be understood that the same referencenumerals are used throughout the Figures to indicate the same or similarparts.

Existing fall detection systems typically consist of two phases. In afirst phase, values for parameters that may indicate the likelihood of afall are obtained for an event from data from sensors such asaccelerometers and barometric sensors. Other sensors may includegyroscopes, magnetometers, ultrasound sensors, radar sensors, skinconductivity sensors and pulse (photoplethysmography, PPG) sensors, forexample. The parameters may include any of height, height change,orientation, orientation change, velocity, duration and impact. Otherexample values include distance to ground (or ceiling) and physiologicalquantities such as heart rate, respiration rate, voice quality (anxiety,specific utterances) and other sounds. Events are typically definedusing a trigger, which monitors at least one trigger parameter: an eventis considered to have occurred at the point at which the value of atrigger parameter exceeds or falls below a corresponding predeterminedtrigger decision value (or when a change of a value is greater than apredetermined change). For example, an event may be detected when thebarometer indicates a height drop of more than 50 cm, or theaccelerometer indicates an acceleration of more than 2*g. Such designstypically make use of advanced machine learning techniques, e.g. deeplearning. In those approaches, a (deep) neural network directly takesthe sensor signals (samples) as an input. The window of samples might becentred around a trigger, but the window may also move over the inputnodes of the neural net (i.e. all samples shift one or more nodesfurther; the latest samples dropping off and new samples entering).

In a second phase, a classifier (e.g. a processing device) takes one ormore of the obtained parameter values from the event and outputs a valueindicating or predicting whether or not the event is a fall. In somecases, the obtained parameter value(s) are used to calculate aprobability that a fall has occurred, e.g. using a fall eventprobability distribution, and the classifier applies a decision to thecalculated probability to decide whether that probability is predict,with a certain degree of accuracy, whether or not a fall has occurred.For example, the output of a neural net can be treated as a probabilityvalue, and by comparison with a decision value, the event (sample set atthe input window) is determined to be a fall or non-fall event. Analternative approach may be determine a likelihood ratio betweenlikelihood an event is a fall and likelihood the event is a non-fall. Inthis case, classification of an event also employs a decision to decidewhether a given event is a fall or a non-fall.

According to a concept of the invention, there is proposed a method andsystem for distinguishing between a fall event and a non-fall-event fora plurality of subjects. The concept includes obtaining first and secondfalse fall detection rates for first and second sub-groups of theplurality of subjects, respectively. Based on the first and second falsefall detection rates, first and second decision values for the first andsecond sub-groups, respectively, are then determined. In this way, thedecision values may be adapted per sub-group.

Embodiments are at least partly based on the realization that a propertyor characteristic of a subject may affect the false detection rate. Byseparating the monitored population of subjects into a pluralitysub-groups (e.g. two or more sub-groups) based on such aproperty/characteristic, a decision value used to distinguish between afall event and a non-fall-event may be adapted to each of a plurality ofsub-groups. In this way, a property/characteristic specific to a groupof monitored subjects (which may influence a false fall detection rate)may be accounted for, thereby providing for adjusted or tailored fallevent classification that leads to improved fall detection accuracy.

Illustrative embodiments may, for example, be employed in (or inconjunction with) fall detectors and/or fall detection systems. Suchdetectors and/or systems may be used in PERS systems at a subject's homeor in specialized accommodation such as Senior Living Facilities, inhospital products, and/or using apps designed for smartwatches.

Proposed is an approach for enabling the dynamic modification of adecision value to a group of subjects having the same property orcharacteristic. Such proposals may thus facilitate simple adaptation ofa decision value to cater for a specific property or characteristic of asub-group of subjects that can influence a false fall detection rate forthat sub-group, and this may for example be done in consideration of atarget false detection rate and/or minimum detection probability.

Embodiments of the present invention may therefore be directed towardimproving the accuracy and/or functionality of a fall detection systems.

By way of example only, illustrative embodiments may be utilized in manydifferent types of clinical, medical or subject-related environments,such as a hospital, doctor's office, ward, care home, person's home,etc.

As mentioned above, proposed concepts are at least partly based on therealization that a property or characteristic of a subject may affectthe false fall detection rate. Purely by way of example, one suchproperty of a subject may be location of the subject. To demonstratethis, FIGS. 1 and 2 illustrate exemplary data demonstrating howlocation-specific factors may contribute to false fall detection rate.

FIG. 1 depicts a variation in false fall detection rate over time forfour different states in America (namely: NY, NJ, AL and FL). Time, inmonths, is represented on the horizontal (X-) axis, and the number offalse fall detections per month (i.e. false fall detection rate) isrepresented on the vertical (Y-) axis.

From FIG. 1, it can be observed that the false fall detection rate canchange significantly over time. Moreover, the false fall detection ratepatterns are similar between neighbouring states, but differ betweenstates that are distant.

This is also confirmed by FIG. 2 which shows a correlation matrixbetween the false alarm rates of the four different states in America(NY, NJ, AL and FL). From FIG. 2 is can be seen that the correlationbetween false alarm rates of neighbouring states is higher. In otherwords, neighbouring states show higher correlation than distant states.

Based on such a realisation that the false fall detection rate can bedependent upon the location of a subject, it is proposed to implementlocation-based monitoring of the false fall detection and modify adecision value for distinguishing between a fall event and anon-fall-event according to subject location. In particular, thedecision value may be defined for a particular location such that theoverall false fall detection remains constant whilst the (average) truepositive rate is improved (compared to when setting one (worst-case)decision value for the whole population). By way of example, this may beachieved by increasing the fall detection decision value in thelocations experiencing a high false fall detection rate, which may thenalso enable the decrease of the decision value for other locations wherethe false fall detection rate is lower for example.

By way of example, an embodiment employing the concept of using locationto groups subject may comprise the following main stages:

(i) Group monitored subjects into sub-groups based on locationinformation;

(ii) Monitor the false fall detection rate of the fall detectionalgorithm for each sub-groups;

(iii) For each sub-group, calculate a fall detection decision valuebased on the false fall detection rate and number of subjects of thesub-group;

(iv) Update the fall detection decision value for all subject accordingto their assigned sub-group.

Although the above-detailed examples employ location information todefine sub-groups, other properties or characteristics of subjects mayinfluence false fall detection rates and thus be used to definesub-groups. For example, in alternative embodiments, a plurality ofsub-groups may be defined based on other subject properties such assubject activity, subject behaviour, subject age, subject gender,subject medical diagnosis, etc.

FIG. 3 illustrates a method for distinguishing between a fall event anda non-fall-event for a plurality of subjects according to a proposedembodiment. Such a method may be used to determine fall detectiondecision values that may be used by a fall detection algorithm in orderto distinguish between a fall event and a non-fall-event for a pluralityof subjects (i.e. to classify a potential fall event as either a fallevent or a non-fall-event).

The method begin with step 310 of obtaining at least first and secondfalse fall detection rates for at least first and second sub-groups ofthe plurality of subjects, respectively.

Here, by way of explanation, a false fall detection rate is a rate atwhich non-fall events are determined by the system to be fall events. Afalse fall detection rate may therefore be thought of as a false alarmrate and may, for example, be measured as the number of false falldetections per unit time (e.g. per hour, per day, per month, etc.) ornumber of false fall detections per subject.

Also, in this example, the first and second sub-groups comprise subjectswith first and second differing values of a property of the plurality ofsubjects, respectively. Put another way, the first sub-group comprises aplurality of subjects each having a first value for a property of asubject (e.g. first geographic location, age between 20-30 years, malegender, etc.) and the second sub-group comprises a plurality of subjectseach having a second, different value for the property (e.g. secondgeographic location, age between 40-50 years, female gender, etc.).

Simply by way of example, the property of the subjects may comprise:subject location; activity; fall risk; age; medical condition; weight;gender, etc. Thus, it will be appreciated that any one of a range ofphysiological, physical, fixed or variable properties may be used todefine sub-groups in the total population of the plurality of subjects.Thus, although the example is described as having two sub-groups, itwill be appreciated that the number of sub-groups may be greater thantwo in other embodiments.

In the embodiment of FIG. 3, the step 310 of obtaining first and secondfalse fall detection rates comprises a set of sub-steps 312 to 318 whichare executed to obtain the first and second false fall detection rates.

Specifically, step 312 comprises identifying the first and secondsub-groups of the plurality of subjects (e.g. by grouping the subjectsinto two sub-groups based on their value of the property). A sub-groupidentifier is then assigned to every subject according to their value ofthe property. In this example embodiment, the subjects are based ontheir geographical location (e.g. obtained from their addressinformation). The resolution of the location information may be, forexample, a zip code, city, state, etc. Of course, it will be appreciatedthat, in other embodiments, the sub-groups may be defined based on otherparameters, such as activity, fall risk, age, gender, medical conditionsor other relevant criteria.

Step 314 then comprises monitoring the plurality of subjects to detectpotential fall events. For each detected potential fall event, thedetected potential fall event is distinguished between being a fallevent and a non-fall-event based on a fall detection algorithm in step316. In other words, step 316 comprises classifying each potential fallevents as either a fall event or a non-fall-event. Using the resultsfrom step 316, first and second false fall detection rates for the firstand second sub-groups, respectively, are determined in step 318.

By way of example, one approach to determining a false fall detectionrate is to use the overall fall detection rate of the algorithm as anapproximation of the false fall detection rate. This may be acceptablebecause, typically, the false fall detection rate is much higher thanthe true fall detection rate.

Another approach to determine a false fall detection rate is to useanother source of information to either confirm or correct the output ofthe fall detection algorithm. This other source of information could,for example, be the data logged by a call centre agent who contacts thesubject whenever a fall is detected and determines, based on a responsefrom the subject, whether or not a fall did happen. Alternatively, oradditionally, detected falls which are cancelled (e.g. marked/indicatedto not be falls) by the subject may be tracked.

Purely by way of example, each false fall event occurrence is pairedwith the sub-group assigned to the subject that generated the false fallevent. The false fall detection rate per group can then be computed bytaking the ratio between the number of false fall event for a givensub-group and the number of active subjects in that sub-group.

In some other embodiments, a confidence interval may be computed for thefalse alarm rate per group. This can be done, for example, using aPoisson distribution to model the number of false alarms per group andapplying a confidence interval formula for the rate parameter.

After determining the first and second false fall detection rates instep 310, the method proceeds to step 320, wherein it is determined ifthe fall detection rates have changed compared to previous or estimatedrates. For example, a change can be detected if the difference betweenthe current and previously estimated false fall detection rate for asub-group exceeds a predetermined decision value. Alternatively,statistical tests can be performed to test for significant differencesin the false fall detection rates using estimated confidence intervals.

If it is determined in step 320 that the false fall detection rates havenot changed (beyond an acceptable amount), the method proceeds to step330 wherein the current decision values employed by the fall detectionalgorithm are maintained (i.e. kept unchanged).

Conversely, if it is determined in step 320 that the false falldetection rates have changed (beyond an acceptable amount), the methodproceeds to step 340 wherein new decision values are determined.

Here, a new decision value is calculated for each sub-group based on theobtained false fall detection rates such that the overall false falldetection rate is bounded (i.e. does not exceed) to a predetermined(acceptable) maximum. Thus, put more simply, step 340 comprisesdetermining, based on the first false fall detection rate, a firstdecision value for distinguishing between a fall event and anon-fall-event of subjects of the first sub-group. Also, step 340comprises determining, based on the second false fall detection rate, asecond decision value for distinguishing between a fall event and anon-fall-event of subjects of the second sub-group.

For instance, determining the first decision value comprises determininga value at which a false fall detection rate for subjects of the firstsub-group is equal to a predetermined rate.

In particular, a sub-group with a higher false fall detection rate willtypically be assigned higher decision values.

In this example, the new decision values are calculated based on aconstrained optimization procedure. The constraints on the decisionvalues are introduced in the optimization to ensure a minimum detectionprobability for all groups.

By way of example, the optimization may be implemented in the followingway:

For ease of notation, it is assumed that there are only two sub-groupsof subject, namely a first sub-group A containing N_(A) subjects and asecond sub-group B comprising N_(B) subscribers, respectively, whereboth N_(A) and N₉ are integers greater than or equal one. Firstsub-group A has a measured false fall detection rate FA_(A) and thesecond sub-group B has a measured false fall detection FA_(B), forpredetermined initial decision value th_(A) and th_(B).

A predetermined ROC curve (Receiver Operating Characteristic) isavailable. It is obtained, for example, by analysing laboratory(simulated) falls. The ROC curve is a functionFP(th),TP(th)=ROC(th)

that takes as input a detection decision value th and returns as outputa FP(th),TP(th) pair. Note that FP(th),TP(th) are the false positive andtrue positive rates per triggered event given decision value th. Withthe ROC curve, and given the measured/observed false alarm rate FA_(G),it is possible to estimate the event trigger rate Trg_(G) (the rate ofoccurrences of events that trigger the fall detection algorithm) foreach group

${{Trg}_{G} = \frac{{FA}_{G}}{{FP}\left( {th}_{G} \right)}},{{{with}\mspace{14mu} G} \in {\left\{ {A,B} \right\}.}}$

Finally, the new decision values are found by solving the followingoptimization problem:

${{th}_{A_{new}}{th}_{B_{new}}} = {\underset{{th}_{1},{th}_{2}}{\arg\;\min}\left( {{N_{A}*{Trg}_{A}*{{FP}\left( {th}_{1} \right)}} + {N_{B}*{Trg}_{B}*{{FP}\left( {th}_{2} \right)}}} \right)}$subject  to  TP(th₁), TP(th₂) > TP_(min).where Trg_(G) are determined as given by the previous equation.

The optimization can also be formulated in a different but similarmanner as the maximization of the detection probability while enforcinga constraint on the false fall detection rate.

After determining new first and second decision values in step 340, themethod proceeds to step 350. In step 350, the newly-determined first andsecond decision values are associated with the first and secondsub-groups, respectively. By way of example, this may entail updatingdecision values employed by subjects' fall detection devices and thosestored in a database for example.

Although, in the above-described embodiment of FIG. 3, the step 310 ofobtaining first and second false fall detection rates for first andsecond sub-groups is undertaken by monitoring the subjects anddetermining the first and second false fall detection rates, it will beappreciated that, in other embodiments, the first and second false falldetection rates for first and second sub-groups may simply be obtainedfrom a separate and/or remotely-located information source (e.g. a falldetection system, database of fall cases, subject monitoring system,etc.). In this way, such alternative embodiments may be configured tosimply receive information about fall detection rates for a monitoredpopulation of subjects, and then employ the received information todetermine decision values for different sub-groups of the monitoredpopulation. An example could employ a look-up table, in which for givenweather conditions/forecasts, a decision value is advised. Further, forweather conditions, the measured false fall detection rate in onelocation (e.g. state) may be used to predict the false fall detectionrate in the neighboring state(s) to which a weather event (e.g. storm ismoving). In this way, a determined decision value for a state may beused to update a decision value for a neighbouring to which the weatheris travelling.

Also, it is noted that, from the above equations, the false falldetection rate (i.e. false alarm rate FA) can be expressed as Trg*FP.Thus, the false fall detection rate can be obtained ‘indirectly’ fromthe Trg*FP product and not by measuring an actual value (e.g. at thefall detector's output). The process of obtaining false fall detectionsrate can be undertaken ‘indirectly’ (i.e. calculated or determined fromother values, such as calculated from a trigger rate or predicted from aneighbouring state for example.)

Further, in another embodiment, the step 320 of determining if the falldetection rates have changed may be omitted and the decision valuealways updated. In the case that the decision value do not changesignificantly, such updating may be effectively unnoticeable.

Strictly, for the test whether the FA-rate (decision values) havechanged, a comparison value is needed. This is the outcome (330 and350). Test can be on FA-rate per subgroup, or on effective total rate.

By way of further illustration of the proposed concept(s), a system fordetecting a fall of a subject according to an embodiment will be now bedescribed with reference to FIG. 4.

FIG. 4 illustrates a system 400 for detecting a fall of a subject. Thesystem 400 comprises one or more sensors 410 and a system 420 fordistinguishing between a fall event and a non-fall-event for a pluralityof subjects according to an embodiment of the invention.

The one or more sensors 410 are configured to obtain movement dataresponsive to the subject's movement. The one or more sensors mayinclude at least one of an accelerometer, a barometric sensor, a tiltswitch, a vibration sensor and a gyroscope. The one or more sensors maybe part of a wearable device, for example, a smartwatch, a pendant wornaround the neck or wrist, a pocket-carried device, an ear-worn device,etc.

The system 420 for distinguishing between a fall event and anon-fall-event for a plurality of subjects comprises an interfacecomponent 430, a decision value calculation unit 440 and amicroprocessor 450 (i.e. a data processing unit). By way of example, thesystem 420 may be hosted on a sever (i.e. external and/or remote fromthe one or more sensors), and it may also be hosted in a distributedmanner.

The interface component 430 is adapted to receive movement data from theone or more sensors 410 (via the microprocessor 450), and to detectevents that occur in the movement data. An event may be detected when adefined trigger parameter (which may be a combination of parameters ofthe movement data) changes by more than a predetermined amount or when avalue of the trigger parameter breaches a predetermined decision value.Other methods, including machine-learning and deep-learning approaches,would be apparent to the skilled person.

Further, the interface component 430 is adapted to obtain first andsecond false fall detection rates for first and second sub-groups of aplurality of subjects, respectively (e.g. from the cloud or a centraldata repository). As with the example of FIG. 3, the first and secondsub-groups comprise subjects with first and second differing values of aproperty. In this example, the property comprises subject age, wherein asubject is deemed to have the first value of age if it has an age withinthe range 10-30 years and wherein a subject is deemed to have the secondvalue of age if it has an age within the range 40-60 years.

More specifically, the interface component 430 of this example comprisesa grouping component 460 configured to identify first and secondsub-groups of the plurality of subjects. Put another way, the interfacecomponent groups subjects in sub-groups based on subject age.

The interface component 430 also includes a monitoring component 470that is configured to monitor the plurality of subjects to detectpotential fall events. Here, the monitoring component 470 receivesmovement data from the one or more sensors 410 (via the microprocessor450), and detect potential fall events that occur in the movement data.

A classification component 480 of the interface component 430 isconfigured, for each detected potential fall event, to distinguishbetween the detected potential fall event being a fall event and anon-fall-event based on a fall detection algorithm employing a decisionvalue. Put another way, the classification component 480 is configuredto classify potential fall events as either a fall event and anon-fall-event, and this employs a threshold-based algorithm.

The interface component 430 also includes a processing component 490configured to determine first and second false fall detection ratesbased on the identified first and second sub-groups and the results ofdistinguishing the potential fall events between being fall events andnon-fall-events (provide by the classification component 480).

The first and second false fall detection rates obtained by theinterface component are provided to the decision value calculation unit440. The decision value calculation unit 440 is adapted to determine newvalues for the first and second decision values based on the first andsecond false fall detection rates, respectively.

More specifically, the decision value calculation unit 440 is adapted todetermine, based on the first false fall detection rate, a firstdecision value for distinguishing between a fall event and anon-fall-event of subjects of the first sub-group. In doing so, thedecision value calculation unit 440 is adapted to determine a value atwhich a false fall detection rate for subjects of the first sub-group isequal to a predetermined rate, e.g. a rate that maintains apredetermined minimum detection probability value (i.e. a minimumsensitivity).

Further, the decision value calculation unit 440 is adapted todetermine, based on the second false fall detection rate, a seconddecision value for distinguishing between a fall event and anon-fall-event of subjects of the second sub-group. Here, the decisionvalue calculation unit 440 is adapted to determine a value at which afalse fall detection rate for subjects of the second sub-group is equalto the predetermined rate, such as the rate that maintains apredetermined minimum detection probability value (i.e. a minimumsensitivity).

The newly-determined decision values are passed from the decision valuecalculation unit 440 to the microprocessor 450. Based on movement datafrom the one or more sensors 410, the microprocessor 450 detects apotential fall event of a subject and determines which sub-group thesubject is a member of. The microprocessor 450 is then configured toclassify the detected potential fall event as a fall event or a non-fallevent by comparing information associated with the fall event with thedecision value for the sub-group the subject is determined to be amember of. The microprocessor 450 thus employs the decision values thatare tailored to each sub-group in order to distinguish between a fallevent and a non-fall event.

It is also noted that the system 400 of FIG. 4 comprises a userinterface 500 that is configured to provide a feedback function. Whenactivated, the feedback function instructs the system 400 fordistinguishing between a fall event and a non-fall-event in order tore-classify a detected fall event as a non-fall event. This userfeedback can then be used to update a false detection rate based on there-classification of the event.

From the above description of exemplary embodiment, it will beunderstood that there is proposed a concept which adapts a falldetection decision value (e.g. threshold value) to different sub-groupsof a monitored or subscriber population, and the sub-groups are definedaccording to a property or characteristic of a subject. By way ofexample, a property may comprise a physiological property or a physicalproperty of subject, and such properties may have fixed or time-varyingvalues.

The fall detection decision values may be determined so as to ensure aminimum detection probability. However, the variation of decision valuesacross different subgroups may be such that an average over them yieldsan improved fall detection accuracy.

It will be understood that the disclosed methods arecomputer-implemented methods. As such, there is also proposed a conceptof a computer program comprising code means for implementing anydescribed method when said program is run on a processing system.

The skilled person would be readily capable of developing a processorfor carrying out any herein described method. Thus, each step of a flowchart may represent a different action performed by a processor, and maybe performed by a respective module of the processing processor.

As discussed above, the system makes use of a processor to perform thedata processing. The processor can be implemented in numerous ways, withsoftware and/or hardware, to perform the various functions required. Theprocessor typically employs one or more microprocessors that may beprogrammed using software (e.g. microcode) to perform the requiredfunctions. The processor may be implemented as a combination ofdedicated hardware to perform some functions and one or more programmedmicroprocessors and associated circuitry to perform other functions.

Examples of circuitry that may be employed in various embodiments of thepresent disclosure include, but are not limited to, conventionalmicroprocessors, application specific integrated circuits (ASICs), andfield-programmable gate arrays (FPGAs).

In various implementations, the processor may be associated with one ormore storage media such as volatile and non-volatile computer memorysuch as RAM, PROM, EPROM, and EEPROM. The storage media may be encodedwith one or more programs that, when executed on one or more processorsand/or controllers, perform the required functions. Various storagemedia may be fixed within a processor or controller or may betransportable, such that the one or more programs stored thereon can beloaded into a processor.

Variations to the disclosed embodiments can be understood and effectedby those skilled in the art in practicing the claimed invention, from astudy of the drawings, the disclosure and the appended claims. In theclaims, the word “comprising” does not exclude other elements or steps,and the indefinite article “a” or “an” does not exclude a plurality. Asingle processor or other unit may fulfill the functions of severalitems recited in the claims. The mere fact that certain measures arerecited in mutually different dependent claims does not indicate that acombination of these measures cannot be used to advantage. A computerprogram may be stored/distributed on a suitable medium, such as anoptical storage medium or a solid-state medium supplied together with oras part of other hardware, but may also be distributed in other forms,such as via the Internet or other wired or wireless telecommunicationsystems. If the term “adapted to” is used in the claims or description,it is noted that the term “adapted to” is intended to be equivalent tothe term “configured to”. Any reference signs in the claims should notbe construed as limiting the scope.

The invention claimed is:
 1. A computer-based method for distinguishingbetween a fall event and a non-fall-event for a plurality of subjects,the method comprising: obtaining first and second false fall detectionrates for first and second sub-groups of the plurality of subjects,respectively, and wherein the first and second sub-groups comprisesubjects with first and second differing values of a property of theplurality of subjects, respectively; determining, based on the firstfalse fall detection rate, a first decision value for distinguishingbetween a fall event and a non-fall-event of subjects of the firstsub-group; and determining, based on the second false fall detectionrate, a second decision value for distinguishing between a fall eventand a non-fall-event of subjects of the second sub-group, wherein thefall detection rate for a sub-group of the plurality of subjects isbased on a ratio between a number of false fall detections for thesub-group and the number of subjects in the sub-group.
 2. The method ofclaim 1, wherein determining the first decision value comprisesdetermining a value at which a false fall detection rate for subjects ofthe first sub-group is equal to a predetermined rate.
 3. The method ofclaim 2, wherein the predetermined rate is defined so as to maintain apredetermined minimum detection probability value.
 4. The method ofclaim 1, wherein the first and second decision values are determined soas to maintain an overall false fall detection rate across the first andsecond groups remains constant whilst the an average true positivedetection rate is increased.
 5. The method of claim 1, furthercomprising: associating the first decision value with each subject ofthe first sub-group; and associating the second decision value with eachsubject of the second sub-group.
 6. The method of claim 1, whereinobtaining first and second false fall detection rates for first andsecond sub-groups of the plurality of subjects comprises: identifyingthe first and second sub-groups of the plurality of subjects; monitoringthe plurality of subjects to detect potential fall events; for eachdetected potential fall event, distinguishing between the detectedpotential fall event being a fall event and a non-fall-event based on afall detection algorithm employing a decision value; and determiningfirst and second false fall detection rates based on the identifiedfirst and second sub-groups and the results of distinguishing thepotential fall events between being fall events and non-fall-events. 7.The method of claim 1, wherein the property of the plurality of subjectscomprises: location; activity; fall risk; age; medical condition;weight; gender; diagnosis; disease; prescription; or userequipment/aids.
 8. A computer program comprising code means forimplementing the method of claim 1 when said program is run on aprocessing system.
 9. A system for distinguishing between a fall eventand a non-fall-event for a plurality of subjects, the system comprising:an interface component adapted to obtain first and second false falldetection rates for first and second sub-groups of a plurality ofsubjects, respectively, and wherein the first and second sub-groupscomprise subjects with first and second differing values of a propertyof the plurality of subjects, respectively; and a decision valuecalculation unit adapted to: determine, based on the first false falldetection rate, a first decision value for distinguishing between a fallevent and a non-fall-event of subjects of the first sub-group;determine, based on the second false fall detection rate, a seconddecision value for distinguishing between a fall event and anon-fall-event of subjects of the second sub-group; and determine avalue at which a false fall detection rate for subjects of the firstsub-group is equal to a predetermined rate.
 10. The system of claim 9,wherein the predetermined rate is defined so as to maintain apredetermined minimum detection probability value.
 11. The system ofclaim 8, further comprising an assignment unit configured to: associatethe first decision value with each subject of the first sub-group; andassociate the second decision value with each subject of the secondsub-group.
 12. The system of claim 8, wherein the interface componentcomprises: a grouping component configured to identify the first andsecond sub-groups of the plurality of subjects; a monitoring componentconfigured to monitor the plurality of subjects to detect potential fallevents; a classification component configured, for each detectedpotential fall event, to distinguish between the detected potential fallevent being a fall event and a non-fall-event based on a fall detectionalgorithm employing a decision value; and a processing componentconfigured to determine first and second false fall detection ratesbased on the identified first and second sub-groups and the results ofdistinguishing the potential fall events between being fall events andnon-fall-events.
 13. A system for detecting a fall of a subject,comprising: one or more sensors for obtaining movement data responsiveto a subject's movement; the system for distinguishing between a fallevent and a non-fall-event for a plurality of subjects of claim 9,further configured to: receive the movement data from the one or moresensors; detect a potential fall event based on the movement data;determine which sub-group the subject is a member of; and classify thedetected potential fall event as a fall event or a non-fall event bycomparing information associated with the fall event with the decisionvalue for the sub-group the subject is determined to be a member of; anda user interface configured to provide a feedback function, which, whenactivated, instructs the system for distinguishing between the fallevent and the non-fall-event to re-classify a detected fall event as anon-fall event, and to update a false detection rate based on there-classification of the event.